The present invention is directed to bicycle components and, more particularly, to a bicycle rear derailleur with a shock absorber that accommodates external forces applied to the derailleur.
FIG. 1 shows a typical bicycle rear derailleur 10 which, for example, may be constructed in accordance with the teachings of U.S. Pat. No. 4,610,644, incorporated herein by reference. Derailleur 10 includes a base member 14 for attaching derailleur 10 to a bicycle frame 16 through an optional fixing member 17, a movable member 18 attached to a chain guide 22 which includes a guide pulley 23 and a tension pulley 24, and links 25, 26 coupled to base member 14 and movable member 18 through connecting pins 28 to form a conventional pantograph mechanism. It is well known in the art that this structure allows chain guide 22 to move from the position shown in solid lines to the position shown in broken lines in response to movement of a control cable (not shown) so as to position chain guide 22 beneath a selected one of a plurality of sprockets S attached to a rear wheel of the bicycle. A threaded shaft 30 is used to fix base member 14 to fixing member 17 (or directly to the bicycle), and a spring (not shown) is disposed within base member 14 to bias base member 14 clockwise (when viewed from the right side) relative to the bicycle. A similar spring (not shown) is disposed within movable member 18 for biasing chain guide 22 clockwise relative to movable member 18 (when viewed from the right side).
A return spring (not shown) is normally provided for biasing chain guide 22 toward a home position, which can be either the position shown in solid lines or the position shown in broken lines. If the return spring biases chain guide 22 toward the position shown in solid lines (so that chain guide 22 is disposed beneath the smallest sprocket S.sub.1), then the derailleur is called a "high normal" derailleur. Pulling the control cable causes such a derailleur to move the chain guide 22 from the position shown in solid lines toward the position shown in broken lines (toward the rear wheel). If the return spring biases chain guide 22 toward the position shown in broken lines (so that chain guide 22 is disposed beneath the largest sprocket S.sub.2), then the derailleur is called a "low normal" derailleur. Pulling the control cable causes such a derailleur to move the chain guide from the position shown in broken lines toward the position shown in solid lines (away from the rear wheel).
The two types of derailleurs (low normal and high normal) differ in how they react to external forces applied to the derailleur, such as when the bicycle falls over or is struck from the side. When an external force toward the rear wheel is applied to a high normal derailleur, the force acts in a direction to compress the control cable. Since the control cable does not resist such compression to any significant extent, the movable member readily moves toward the rear wheel to accommodate the external force. However, when an external force toward the rear wheel is applied to a low normal derailleur, the force acts in the direction to further tension the control cable. In this case the control cable strongly resists movement of the movable member toward the rear wheel, and damage to the cable or derailleur is likely to occur.